1. Field of the Invention
This invention relates to a vertical free surface type pump and more particularly, to a mechanical pump for a liquid metal. The pump includes an emergency cover gas line shut-off system for preventing gas release by solidified liquid metal and further includes an emergency syphon system for discharging an excess volume of liquid metal in a pump casing, to thereby prevent the free surface in the pump casing from rising up to an upper mechanical bearing at the time of failure of the cover gas line. 2. Description of the Prior Art
A vertical, free surface type mechanical pump having a construction which includes the free surface of liquid metal inside a pump casing and in which a cover gas space is disposed over the free surface of the liquid metal is the most ordinary type used as a primary circulating pump for a loop-type, liquid metal-cooled, fast breeder reactor. An example of such a mechanical pump is shown in FIG. 1A. Liquid sodium flows into a substantially cylindrical casing 1 from a suction nozzle 2 at the lower end of the casing 1, obtains a delivery pressure from an impeller 3, and flows out from a delivery nozzle 4. The liquid metal entering an overflow column 6 through an overflow pipe 5 is again returned to the suction nozzle 2. A drive shaft 7 for transmitting the rotating force to the impeller 3 is pivotally supported by a lower hydrostatic bearing 8 and an upper mechanical bearing 9. A mechanical seal 10 is disposed below the mechanical bearing 9 so as to prevent the leakage of a cover gas (i.e. an inert gas) from the casing 1. The inert gas is caused to constantly flow downwards from the mechanical seal 10 in order to prevent the vapor of the liquid metal from rising into the seal and, at the same time, to apply a predetermined cover gas pressure, thereby setting the level of the free surface inside the pump and providing a required suction head necessary for the pump.
In the example of the prior art shown in FIG. 1A, the cover gas is supplied from a gas feed pipe 11 fitted to the lower part of the mechanical seal 10, descends through the gap between a shield plug 12 and the shaft 7, then enters a cover gas space 13 and is recovered through a gas discharge pipe 14 connected to the overflow column 6 and through an exhaust pipe 15. Thus, the cover gas circulation is effected. Accordingly, if the cover gas line or piping for the above-described cover gas circulation is accidentally broken, the free surface inside the pump drastically rises and, at times, it reaches the mechanical seal 10 as well as the mechanical bearing 9, thus causing serious damage to the pump and the leakage of the liquid metal. Even if the free surface does not rise up to the mechanical seal 10, the atmospheric gas at the broken portion of the cover gas line would mix with the cover gas in the pump and would oxidize the liquid metal in the pump.
The free surface inside the pump as well as the free surface inside the overflow column 6 shown in FIG. 1A are those surfaces established when the pump is under normal operation. FIG. 1B shows the changes in the free surfaces inside the pump and inside the overflow column when the pump is used as a primary circulating pump of the primary cooling system of a liquid metal-cooled, fast breeder reactor and the gas feed pipe 11 or exhaust pipe 15 is broken. The ordinate in FIG. 1B corresponds to the height in FIG. 1A and the abscissa represents the elapsed time after the gas line failure. Each curve in FIG. 1B has the following meaning. Reference numeral 20 represents a pump free surface; reference numeral 21 represents an overflow column free surface; reference numeral 22 represents a cover gas pressure in a reactor vessel (represented by the head of the liquid metal); reference numeral 23 represents the upper end position of the gas line 14; reference numeral 24 represents the lower surface position of the shield plug 12; and reference numeral 25 represents the position of the gas feed pipe 11.
In this case, since the cover gas line of the pump is communicated with a cover gas system of a reactor vessel 30 as shown in FIG. 2, the gas pressure inside the pump drops down to the atmospheric pressure within a short period of time if the gas line in the proximity of the pump is broken. On the other hand, since the capacity of a gas space 31 in the reactor vessel 30 is by far greater than the pump, it is known that it takes more than one minute before the cover gas pressure on the side of the reactor vessel drops down to the atmospheric pressure. In the interim, unbalance of the gas pressure develops between the reactor vessel and the pump so that the free surface inside the pump rises due to this pressure difference. In other words, the free surface inside the overflow column 6 rises simultaneously with the failure of the gas line and, when it reaches the same level as the free surface inside the pump, both rise together. However, when the free surface reaches the upper end of the gas line 14, the free surface inside the overflow column 6 can hardly rise any longer and only the free surface inside the pump continues rising. At this point, the cover gas pressure in the reactor vessel 30 has still a high pressure so that the free surface inside the pump reaches the shield plug 12, the mechanical seal 10 and the mechanical bearing 9, thereby not only causing serious damage to the pump but also inviting the leakage of the liquid metal outside the pump. This danger becomes more serious in the case of the cold leg pump arrangement because the cover gas pressure during operation is higher than that of the hot leg pump arrangement.
Incidentally, in FIG. 2, reference numeral 32 represents an outlet nozzle; reference numeral 33 represents a main cooling pipe; reference numeral 34 represents a drain valve; reference numeral 35 represents a drain tank; and refernce numeral 36 represents a cover gas refiner.